1. Field of the Invention
The present invention relates to a layered catalyst composite useful for the treatment of gases to reduce the level of contaminants contained therein. More specifically, the present invention is concerned with catalysts of the type generally referred to as xe2x80x9cthree-way conversionxe2x80x9d or xe2x80x9cTWCxe2x80x9d catalysts. These TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.
2. Related Art
Three-way conversion catalysts have utility in a number of fields including the treatment of exhaust gas streams from internal combustion engines, such as automobile, truck and other gasoline-fueled engines. Emission standards for unburned hydrocarbons, carbon monoxide and nitrogen oxide contaminants have been set by various governments and must be met by older as well as new vehicles. In order to meet such standards, catalytic converters containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. Such catalysts promote the oxidation by oxygen in the exhaust gas stream of unburned hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides to nitrogen.
Known TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum, paladium, rhodium, rhenium and iridium) disposed on a high surface area, refractory metal oxide support, e.g., a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material.
U.S. Pat. No. 4,134,860 relates to the manufacture of catalyst structures. The catalyst composition can contain platinum group metals, base metals, rare earth metals and refractory, such as alumina support The composition can be deposited on a relatively inert carrier such as a honeycomb.
The high surface area alumina support materials, also referred to as xe2x80x9cgamma aluminaxe2x80x9d or xe2x80x9cactivated alumina,xe2x80x9d typically exhibit a BET surface area in excess of 60 square meters per gram (xe2x80x9cm2/gxe2x80x9d), often up to about 200 m2/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. It is disclosed to utilize refractory metal oxides other than activated alumina as a support for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha alumina and other materials are known for such use. Although many of these materials suffer from the disadvantage of having a considerably lower BET surface area than activated alumina, that disadvantage tends to be offset by a greater durability of the resulting catalyst.
In a moving vehicle, exhaust gas temperatures can reach 1000xc2x0 C., and such elevated temperatures cause the activated alumina (or other) support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania alkalie earth metal oxides such as baria, calcia or strontia or rare earth metal oxides, such as ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al., U.S. Pat. No. 4,171,288.
Bulk cerium oxide (ceria) is disclosed to provide an excellent refractory oxide support for platinum group metals other than rhodium, and enables the attainment of highly dispersed, small crystaltites of platinum on the ceria particles, and that the bulk ceria may be stabilized by impregnation with a solution of an aluminum compound, followed by calcination. U.S. Pat. No. 4,714,694 of C. Z. Wan et al., discloses aluminum-stabilized bulk ceria, optionally combined with an activated alumina, to serve as a refractory oxide support for platinum group metal components impregnated thereon. The use of bulk ceria as a catalyst support for platinum group metal catalysts other than rhodium, is also disclosed in U.S. Pat. Nos. 4,727,052 and 4,708,946.
U.S. Pat. No. 4,923,842 discloses a catalytic composition for treating exhaust gases comprising a first support having dispersed thereon at least one oxygen storage component and at least one noble metal component, and having dispersed immediately thereon an overlayer comprising lanthanum oxide and optionally a second support. The catalyst layer is separate from the lanthanum oxide. The noble metal can include platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component can include the oxide of a metal from the group consisting of iron, nickel, cobalt and the rare earths. Illustrative of these are cerium, lanthanum, neodymium, praseodymium, etc. Oxides of cerium and praseodymium are particularly useful as oxygen storage components.
U.S. Pat. No. 4,808,564 discloses a catalyst for the purification of exhaust gases having improved durability which comprises a support substrate, a catalyst carrier layer formed on the support substrate and catalyst ingredients carried on the catalyst carrier layer. The catalyst carrier layer comprises oxides of lanthanum and cerium in which the molar fraction of lanthanum atoms to total rare earth atoms is 0.05 to 0.20 and the ratio of the number of the total rare earth atoms to the number of aluminum atoms is 0.05 to 0.25.
U.S. Pat. No. 4,438,219 discloses an alumina-supported catalyst for use on a substrate. The catalyst is stable at high temperatures. The stabilizing material may be one of several compounds including those derived from barium, silicon, rare earth metals, alkali and alkaline earth metals, boron, thorium, hafnium and zirconium. Of the stabilizing materials barium oxide, silicon dioxide and rare earth oxides which include lanthanum, cerium, praseodymium, neodymium, and others are indicated to be preferred. It is disclosed that contacting them with some calcined alumina film permits the calcined alumina film to retain a high surface area at higher temperatures.
U.S. Pat. Nos. 4,476,246, 4,591,578 and 4,591,580 disclose three-way catalyst compositions comprising alumina, ceria, an alkali metal oxide promoter and noble metals. U.S. Pat. No. 4,591,518 discloses a catalyst comprising an alumina support with components deposited thereon consisting essentially of a lanthana component, ceria, an alkali metal oxide and a platinum group metal.
U.S. Pat. No. 4,591,580 discloses an alumina-supported platinum group metal catalyst. The support is sequentially modified to include support stabilization by lanthana or lanthana rich rare earth oxides, double promotion by ceria and alkali metal oxides and optionally nickel oxide.
Palladium-containing catalyst compositions, see, e.g., U.S. Pat. No. 4,624,940, have been found useful for high temperature applications. The combination of lanthanum and barium is found to provide a superior hydrothermal stabilization of alumina which supports the catalytic component, palladium.
U.S. Pat. No. 4,780,447 discloses a catalyst which is capable of controlling HC, CO and NOx as well as H2S in emissions from the tailpipe of catalytic converter-equipped automobiles. The use of the oxides of nickel and/or iron is disclosed as a hydrogen sulfide gettering-type of compound.
U.S. Pat. No. 4,965,243 discloses a method to improve thermal stability of a TWC catalyst containing precious metals by incorporating a barium compound and a zirconium compound together with ceria and alumina. This is stated to form a catalytic moiety to enhance stability of the alumina washcoat upon exposure to high temperature.
J01210032 (and AU-615721) discloses a catalytic composition comprising palladium, rhodium, active alumina, a cerium compound, a strontium compound and a zirconium compound These patents suggest the utility of alkaline earth metals in combination with ceria, zirconias to form a thermally-stable alumina-supported palladium-containing washcoat.
U.S. Pat. Nos. 4,624,940 and 5,057,483 refer to ceria-zirconia containing particles. It is found that ceria can be dispersed homogeneously throughout the zirconia matrix up to 30 weight percent of the total weight of the ceria-zirconia composite to form a solid solution A co-formed (e.g., co-precipitated) ceria oxide-zirconia particulate composite can enhance the ceria utility in particles containing ceria-zirconia mixture. The ceria provides the zirconia stabilization and also acts as an oxygen storage component. The ""483 patent discloses that neodymium and/or yttrium can be added to the ceria-zirconia composite to modify the resultant oxide properties as desired.
U.S. Pat. No. 4,504,598 discloses a process for producing a high temperature resistant TWC catalyst. The process includes forming an aqueous slurry of particles of a gamma or activated alumina and impregnating the alumina with soluble salts of selected metals including cerium, zirconium, at least one of iron and nickel and at least one of platinum, palladium and rhodium and, optionally, at least one of neodymium, lanthanum, and praseodymium. The impregnated alumina is calcined at 600xc2x0 C. and then dispersed in water to prepare a slurry which is coated on a honeycomb carrier and dried to obtain a finished catalyst.
U.S. Pat. No. 4,587,231 discloses a method of producing a monolithic three-way catalyst for the purification of exhaust gases. First, a mixed oxide coating is provided to a monolithic carrier by treating the carrier with a coating slip in which an active alumina powder containing cerium oxide is dispersed together with a ceria powder and then baking the treated carrier. Next platinum, rhodium and/or palladium are deposited on the oxide coating by thermal decomposition. Optionally, a zirconia powder may be added to the coating slip.
U.S. Pat. No. 4,923,842 discloses a catalytic composition for treating exhaust gases comprising a first support having dispersed thereon at least one oxygen storage component and at least one noble metal component, and having dispersed immediately thereon an overlayer comprising lanthanum oxide and optionally a second support. The layer of catalyst is separate from the lanthanum oxide. The noble metal can include platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component can include the oxide of a metal selected from the group consisting of iron, nickel, cobalt and the rare earths, e.g., cerium, lanthanum, neodymium, etc.
U.S. Pat. No. 5,057,483 discloses a catalyst composition suitable for conversion of exhaust gases emanating from an internal combustion engine and includes a catalytic material disposed in two discrete coats on a carrier. The first coat includes a stabilized alumna support on which a first platinum catalytic component is disperseand The first coat also includes bulk ceria and may also include bulk iron oxide, a metal oxide, e.g., bulk nickel oxide, and one or both of baria and zirconia dispersed throughout as a thermal stabilizer. The second coat, which may comprise a top coat overlying the first coat, contains a co-formed, e.g., co-precipitated rare earth oxide-zirconia support on which a first rhodium catalytic component is dispersed, and a second platinum catalytic component dispersed thereon. The second coat may also include a second rhodium catalytic component and optionally, a third platinum catalytic component, dispersed as an activated alumina support.
WO 95/35152 discloses a layered catalyst composite of the TWC type. The composite contains a first layer and a second layer. The first layer comprises a fist support, at least one first palladium component, optionally a minor amount of a platinum component, optionally a first oxygen storage component, optionally a zirconium component, optionally at least one alkaline earth metal component and optionally at least one lanthanum and/or neodymium component. The second layer comprises a second support, a second platinum component, a rhodium component, a diluted second oxygen storage component and optionally a zirconium component.
It is a continuing goal to develop a three-way conversion catalyst system which is inexpensive and stable at the high temperatures generated by an internal combustion engine. At the same time, the system should have the ability to oxidize hydrocarbons and carbon monoxide while reducing nitrogen oxides to nitrogen.
The present invention relates to a layered catalyst composite of the type generally referred to as a three-way conversion catalyst. These TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. The relative layers of the catalyst composite and the specific composition of each such layer provide a stable, economical system. This enables the enhanced oxidation of hydrocarbons and carbon monoxide as well as effective conversion of nitrogen oxide compounds to nitrogen even where palladium is the only noble metal component in the composite.